1. Field of the Invention
The present invention relates to a solid state image sensor device, and more particularly to a solid state image sensor device and a method for fabricating the same, which has a burled type photodiode for a photoelectric conversion section.
2. Description of the Related Art
In a conventional solid state image sensor device of the type to which the present invention relates, an N.sup.- -P junction photodiode is used in its photoelectric conversion section. With the N.sup.- -P Junction photodiode, due to the generation of a current based on Silicon-Silicon dioxide interface electron energy when the surface of the N.sup.- -region is completely depleted, there is an increase in a noise component which is not due to photoelectric conversion. This current is called a dark current and it causes the signal/noise ratio (S/N ratio) to be deteriorated under a condition of low illumination.
As a method for decreasing the dark current, Teranishi et al has proposed in the Japanese Patent Application Kokai No. Sho 57-62557 (A), a burled photodiode for use in a photoelectric conversion section of a solid state image pickup device, in which a highly doped P.sup.+ -type region is formed on a surface of an N-type region of the photodiode and, by fixing the potential thereof to the reference potential, the P.sup.+ -type region at the surface is confined to the non-depletion region even when the N-type region is completely depleted and the cause for the generation of the dark current is eliminated.
FIGS. 1 to 8 are sectional views of a cell section of a conventional solid state image sensor device which uses a buried type photodiode in a photoelectric conversion section, for explaining the sequential fabrication steps thereof.
First, a P-type well layer 2 is formed on an N-type semiconductor substrate 1 (FIG. 1).
Then, a silicon oxide film 3 and a silicon nitride film 4 are sequentially grown on the P-type well layer 2. A photoresist 5a is applied on the silicon nitride film 4 and is subjected to exposure and development processes. Thereafter, the silicon nitride film 4 at a portion to become the photoelectric conversion section is selectively removed using a plasma-etching process (FIG. 2).
Next, with the photoresist 5a and the silicon nitride film 4 used as masks, ion implantation is performed whereby an N-type region 6 of the photoelectric conversion section is formed (FIG. 3).
Then, the photoresist 5a and the silicon nitride film 4 are removed using a wet-etching process. Thereafter, a portion other than the signal electron transfer section is covered by a photoresist 5b using a photolithography technique. With the photoresist 5b used as a mask, ion implantation is performed whereby an N-type region 8 of the signal electron transfer section is formed (FIG. 4).
Next, by using a photolithography technique, a photoresist 5c is applied to cover the signal electron read-out section, the photoelectric conversion section and the signal electron transfer section and, by using this photoresist 5c as a mask, ion implantation is performed whereby a P.sup.+ -type region 9 to serve as an element isolation portion is formed (FIG. 5).
Thereafter, the photoresist 5c is removed and, after the silicon oxide film 3 is etched away, a first gate insulating film (not shown) is formed by a thermal oxidation process. Then, a polycrystalline silicon film is deposited by a low-pressure chemical vapor deposition (LPCVD) method and, by using a photolithography technique and a dry-etching method, a first polycrystalline silicon electrode (not shown) for transferring signal electrons is formed. With the first polycrystalline silicon electrode used as a mask, the first gate insulating film is etched away and a second gate insulating film 10 is formed by a thermal oxidation process newly performed. Then, by using the same technique and method as used for forming the first polycrystalline silicon electrode, there is formed a second polycrystalline silicon electrode 11 for reading-out signal electron and for transferring signal electron from the photoelectric conversion section to the signal electron transfer section (FIG. 6).
Further, the second gate insulating film 10 which is exposed is etched back and, after a silicon oxide film 12 is formed by a further thermal oxidation process, ion implantation is performed with the polycrystalline silicon electrode 11 used as a mask whereby a shallow P.sup.+ -type region 13 is formed in a surface area of the photoelectric conversion section (FIG. 7).
Lastly, after an interlayer insulating film 14 is formed, contact holes (not shown) are formed therein on the first and second polycrystalline silicon electrodes, and a metal film serving both as a light-shielding film 15 and a wiring film (not shown) is formed using a photolithography technique and a dry-etching method, whereby a conventional solid state image sensor device having a buried photodiode at its photoelectric conversion section is obtained (FIG. 8).
In the above explained conventional solid state image sensor device which has a buried photodiode as a photoelectric conversion section, since the N-type region 6 of the photoelectric conversion section and the P.sup.+ -type region at the surface layer thereof are formed by processes different from each other, it is difficult to confine, within a predetermined range, the overlapping dimension between the N-type region of the photoelectric conversion section and the polycrystalline silicon electrode, due to any misalignment which may occur in the course of photolithography process or to dimension deviations of the electrodes.
For example, in the case where the overlapped dimension shown by "X" in FIG. 9A is large, a deep potential well is formed under the polycrystalline silicon electrode 11 as shown with A in the graph of FIG. 9B. Conversely, in the case where this overlapped dimension "X" is small as shown with B in FIG. 10A, a potential barrier is formed under the polycrystalline silicon electrode 11 as shown in the graph of FIG. 10B. It is noted that, in either of the above cases, smooth transfer of signal electrons from the photoelectric conversion section to the signal electron transfer section is hindered, which is a disadvantage in the conventional arrangement.